The formulation of synthetic or mineral oil based lubricants typically includes an additive package incorporating a variety of chemicals to improve or protect lubricant properties in application specific situations, particularly internal combustion engine and machinery applications. The more commonly used additives include oxidation inhibitors, rust inhibitors, antiwear agents, pour point depressants, detergent-dispersants, viscosity index (VI) improvers, foam inhibitors and the like. This aspect of the lubricant arts is specifically described in Kirk-Othmer "Encyclopedia of Chemical Technology", 3rd edition, Vol. 14, pp 477-526, incorporated herein by reference. The inclusion of additives in hydrocarbon lubricants provides a continuing challenge to workers in the field to develop improved additives of increased compatibility with the lubricant. Superior additives, while contributing their inherent attribute to the formulation, must do so while maintaining or improving upon the composite thermal and oxidative stability of the lubricant formulation.
The low temperature flow characteristic of hydrocarbon lubricants are typically improved by adding pour point depressants (PPD) to the formulation. At low temperatures, these additives modify the shape and size of the precipitating waxy hydrocarbon crystal to slow agglomeration and lower the effective pour point temperature of the lubricant formulation. Currently, preferred pour point depressants include polymethacrylates and ethylene-vinyl ester polymers. However, hydrocarbon based pour point depressants are known.
Polyalphaolefin (PAO) pour point depressants are described by Chong-Xiang Xiong in "The Structure and Activity of Polyalphaolefins as Pour Point Depressants", published in the Journal of the Society of Tribologists and Lubrication Engineers, March, 1993, pp 196-200. The PPD is prepared by polymerization of slack wax-derived C.sub.7 -C.sub.20 alphaolefins using Ziegler-Natta catalyst. It is reported that PAO pour point depressant activity depends on average side chain length and on the distribution of the side chain length. Base oil characteristics influence the effectiveness of specific PAO pour point depressants.
It is also known that the low temperature flow properties of waxy distillate fuels can be improved by employing wax crystal modifiers as additives to fuels in a manner functionally similar to waxy lube PPD. The use of such additives to distillate fuels avoids the more costly step of deep dewaxing of the distillate feedstock.
One class of lubricants of particular interest in the present invention is synthetic lubricants obtained by the oligomerization of olefins, particularly C.sub.3 -C.sub.20 alpha olefins. Catalytic oligomerization of olefins has been studied extensively. Known olefin oligomerization catalysts include the Ziegler-Natta type catalysts and promoted catalysts such as BF.sub.3 or AlCl.sub.3 catalysts. U.S. Pat. No. 4,613,712 for example, teaches the preparation of isotactic alpha-olefins in the presence of a Ziegler type catalyst. Other coordination catalysts, especially chromium on a silica support, are described in the art.
Recently, novel lubricant compositions (referred to herein as HVI-PAO and the HVI-PAO process) comprising polyalphaolefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. Pat Nos. 4,827,064 and 4,827,023, incorporated herein by reference in their entirety. The process comprises contacting C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone whereby high viscosity, high VI liquid hydrocarbon lubricant is produced having low methyl to methylene branch ratios of less than 0.19 and pour point below -15.degree. C. The process is distinctive in that little isomerization of the olefinic bond occurs compared to known oligomerization methods to produce polyalphaolefins using Lewis acid catalyst.
U.S. Pat. No. 5,146,021 to Jackson, et al. discloses lube compositions of HVI-PAO with mineral oil and polyolefins wherein oligomers from mixtures of C.sub.6 -C.sub.20 alphaolefins are employed to provide high VI additives and shear stability. However, the patent does not claim or disclose the use of mixtures containing a high proportion of C.sub.18 + alpha olefins to produce improved pour point depressants. U.S. Pat. No. 5,157,177 to Pelrine et al. discloses the oligomerization process relevant to the preparation of the foregoing HVI-PAO compositions. The compositions and process disclosed in these patents encompass polymer compositions that contain non-waxy components. The polymers are useful as lubricants with low pour point.
The object of the present invention is the production of novel lubricant additive hydrocarbon compositions that are highly effective as pour point depressants and/or combined pour point depressant and viscosity index improver (VII) produced by catalytic oligomerization of 1-alkenes.
Another object of the present invention is to provide an improved process for the production of pour point depressants by oligomerization of 1-alkenes using silica supported reduced chromium oxide catalyst.
Yet another object of the invention is to provide a pure hydrocarbon additive for distillate fuels that is effective in modifying wax crystal growth at low temperatures.